I had always heard it's in the 300 range. Phil Plait did an analysis back in the day using "display held one foot away" as the metric and determined that "average eyesight" could resolve 286 PPI. "Perfect eyesight" would bump it to 477 PPI.
I remember that 800 number from one of the books about graphics and rendering. Forgot which one, probably Glassner's. So it might be more of a theoretical/ultimate threshold than anything. Also, it's not about resolving individual pixels but removing aliasing effects due to signal /picture density.
http://blogs.discovermagazine.com/badastronomy/2010/06/10/re...